2.4.3 Window Type

A window describes the portion of the terminal screen that Emacs
uses to display a buffer. Every window has one associated buffer, whose
contents appear in the window. By contrast, a given buffer may appear
in one window, no window, or several windows.

Though many windows may exist simultaneously, at any time one window
is designated the selected window. This is the window where the
cursor is (usually) displayed when Emacs is ready for a command. The
selected window usually displays the current buffer, but this is not
necessarily the case.

Windows are grouped on the screen into frames; each window belongs to
one and only one frame. See Frame Type.

Windows have no read syntax. They print in hash notation, giving the
window number and the name of the buffer being displayed. The window
numbers exist to identify windows uniquely, since the buffer displayed
in any given window can change frequently.